Boundary Shear Stress in Compound Channel

Abstract

Magnitude of flood prediction is the fundamental for flood warning, determining the development for the present flood-risk areas and the long-term management of rivers. Discharge estimation methods currently employed in river modeling software are based on historic hand calculation formulae such as Chezy’s, Darcy-Weisbatch or Manning’s equation. More recent work has provided significant improvements in understanding and calculation of channel discharge. This ranges from the gaining knowledge to interpretation of the complex flow mechanisms to the advent of computing tools that enable more sophisticated solution techniques.
When the flows in natural or man made channel sections exceed the main channel depth, the adjoining floodplains become inundated and carry part of the river discharge. Due to different hydraulic conditions prevailing in the river and floodplain, the mean velocity in the main channel and in the floodplain are different. Just above the bank-full stage, the velocity in main channel is much higher than the floodplain. Therefore the flow in the main channel exerts a pulling or accelerating force on the flow over floodplains, which naturally generates a dragging or retarding force on the flow through the main channel. This leads to the transfer of momentum between the main channel water and that of the floodplain. The interaction effect is very strong at just above bank full stage and decreases with increase in depth of flow over floodplain. The relative “pull” and “drag” of the flow between faster and slower moving sections of a compound section complicates the momentum transfer between them. Failure to understand this process leads to either overestimate or underestimate the discharge leading to the faulty design of channel section. This causes frequent flooding at its lower reaches.